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It is not a mountain range in the normal sense, because it was formed as a single mountain called Koʻolau Volcano (koʻolau means "windward" in Hawaiian, cognate of the toponymTokelau). What remains of Koʻolau is the western half of the original volcano that was destroyed in prehistoric times when the entire eastern half—including much of the summit caldera—slid cataclysmically into the Pacific Ocean. Remains of this ancient volcano lie as massive fragments strewn nearly 100 miles (160 km) over the ocean floor to the northeast of Oʻahu. Kāneʻohe Bay is what remains of the ancient volcano's summit caldera after the slide. The modern Koʻolau mountain forms Oʻahu's windward coast and rises behind the leeward coast city of Honolulu — on its leeward slopes and valleys are located most of Honolulu's residential neighborhoods.

View of Koʻolau Range from the top. Kāneʻohe is visible on the right side.

The volcano is thought to have first erupted on the ocean floor more than 2.5 million years ago. It eventually reached sea level and continued to grow in elevation until about 1.7 million years ago, when the volcano became dormant. The volcano remained dormant for hundreds of thousands of years, during which time erosion ate away at the initially smooth slopes of the shield-shaped mountain; and the entire mass subsided considerably. The highest elevation perhaps exceeded 3,000 meters (9,800 ft); today, the summit of the tallest peak, Puʻu Konahuanui is only 945 meters (3,100 ft).

After hundreds of thousands of years of dormancy, Koʻolau volcano began to erupt again. Some thirty eruptions over the past 500,000 years or so have created many of the landmarks around eastern Oʻahu, such as Diamond Head, Koko Head (Hanauma Bay), Koko Crater, Punchbowl Crater, Tantalus, and Āliapaʻakai, and are collectively known as the Honolulu Volcanic Series.[4] Geologists do not always agree on the dates of these more recent eruptions, some dating them to around 32,000 years ago, others to as recently as 10,000 years ago. Geologists believe that there is at least a remote possibility that Koʻolau volcano will erupt again.